The Ionized calcium-Binding Adapter molecule 1 (Iba1), also known as Allograft Inflammatory Factor 1 (AIF-1), is a 17 kDa cytokine-inducible protein, produced by activated macrophages during chronic transplant rejection and inflammatory reactions in Vertebrates. In mammalian central nervous system (CNS), Iba1 is a sensitive marker associated with activated macrophages/microglia and is upregulated following neuronal death or brain lesions. The medicinal leech Hirudo medicinalis is able to regenerate its CNS after injury, leading to a complete functional repair. Similar to Vertebrates, leech neuroinflammatory processes are linked to microglia activation and recruitment at the lesion site. We identified a gene, named Hmiba1, coding a 17.8 kDa protein showing high similarity with Vertebrate AIF-1. The present work constitutes the first report on an Iba1 protein in the nervous system of an invertebrate. Immunochemistry and gene expression analyses showed that HmIba1, like its mammalian counterpart, is modulated in leech CNS by mechanical injury or chemical stimuli (ATP). We presently demonstrate that most of leech microglial cells migrating and accumulating at the lesion site specifically expressed the activation marker HmIba1. While the functional role of Iba1, whatever species, is still unclear in reactive microglia, this molecule appeared as a good selective marker of activated cells in leech and presents an interesting tool to investigate the functions of these cells during nerve repair events. © 2014 Wiley Periodicals, Inc. Develop Neurobiol, 2014.